Referring to FIG. 1, a block diagram of a circuit 10 illustrating a half-duplex bus architecture is shown. The circuit includes a bi-directional bus with a plurality of half-duplex channels that only support half-duplex communication. The circuit 10 can connect a first circuit (card) 12 to a second circuit (card) 14. The circuits 12 and 14 can exchange data via transmission lines (pairs of wires) 16. Each pair of wires 16 can be used to transmit or receive data at a rate of 77.75 MB/s. At each end of 20 each pair of wires 16, an output buffer 18 and an input buffer 20 are connected together. Connecting the input buffer 20 and output buffer 18 together can result in a bus contention when the output buffers (transmitters) 18 at both ends of the transmission lines 16 are active. Depending on buffer (driver) type, additional circuits can be required to prevent the output buffers 18 from self-destructing. The addition of the input buffer 20 in parallel with the output buffer 18 can increase capacitance of the source and destination end of the transmission lines 16. The increased capacitance can affect the termination-impedance of the transmission lines 16.
The conventional half-duplex bus can have the following disadvantages: 1. bus contention during hot insertion of cards (e.g., both output buffers 18 of a channel 16 can be ON at the same time); 2. half-duplex operation (i.e., only one card can transmit at one time); 3. additional components can be required to prevent the destruction of the output buffers 18; 4. termination at both ends of the transmission line places increased load and current-drive requirements on the enabled output buffer 18; 5. bus turn-around can require phase lock loops (PLLs) to re-acquire lock, therefore, slowing data exchange.
A conventional method for providing a full-duplex bi-directional bus is to a use a hybrid network to allow simultaneous bi-directional transmission of data across the same pair of wires. The drawback to a hybrid circuit is that the hybrid networks can require echo cancellation circuits which can require a 6 dB or better signal to noise ratio (SNR). The hybrid circuit can also require receivers with greater sensitivity or larger source amplitude for valid transmission. The greater sensitivity and large source amplitude can result in greater power dissipation, higher complexity, and possibly greater radiated emissions.